1. Field of the Invention
This invention relates to thin film magnetic write head fabrication procedures in general and, more specifically, to a method of forming and trimming an upper pole piece for such a write head.
2. Description of the Related Art
Thin film magnetic write heads are used to encode magnetically stored information on moving magnetic media such as tapes and discs. In the simplest terms, such a head consists of two pole pieces separated at the air-bearing surface (ABS) by a narrow gap (the write gap) and yoked together behind the ABS to form an approximately horseshoe-shaped assembly. A conductive coil is patterned between the pole pieces which, when electrically energized, induces a magnetic field between the poles which fringes across the gap. It is this induced field that encodes small magnetic regions in the moving medium.
The rapid changes in the state of the art have necessitated continual improvements in the area density of information that can be magnetically encoded and decoded in the moving medium. For a disk, this area density is a product of the number of recording tracks per mm measured radially, and the number of flux reversals per mm along the track, measured tangentially. With the development of the extremely sensitive magneto-resistive read heads, methods for improving the area density are now focussing on extending the limits of the inductive writing technology.
There are several approaches to improving the writing technology, one of which is to narrow track widths and thereby increase the number of tracks per mm. This approach requires that the writing tip of the magnetic pole assembly, including its write gap, be made as narrow as possible. In addition, the fringing fields extending beyond the width of the write gap should be minimized so that writing does not also occur on portions of the recording media away from the selected track. Such fringe field minimization requires the write gap to be formed in a symmetric pole piece. One way of forming such narrow, symmetric pole pieces and write gaps is to first form them with a relatively wide shape and then trim the shape to the desired width by means of an etching process, which, typically is a series of ion-beam etches (IBE) or milling process. These etching processes are time consuming and lead to undesirable width discontinuities between various portions of the pole piece. A prior art method of producing a trimmed upper pole piece as presently practiced by the inventors is taught by Chen et al. (U.S. Pat. No. 6,243,939) and is more briefly described herein with reference to FIGS. 1a-1g. Referring to FIG. 1a there is shown a schematic cross-sectional view of a write gap (WG) layer (20) formed on a lower shield (10) of a read/write head. As is typical in the prior art, the material of choice for forming a write gap layer is alumina. Referring to FIG. 1b, there is shown the configuration of FIG. 1a with the addition of a seed layer (30), which will form the basis for subsequent deposition of a plated portion of the pole piece. Referring next to FIG. 1c, there is shown a monolithic plated pole portion (40) formed on the seed layer. The monolithic plated pole portion is plated to a uniform width within a form (not shown) in a manner well known by practitioners of the art. Referring to FIG. 1d, there is shown the fabrication of FIG. 1c wherein a substantial portion of the seed layer (30) has been removed by an ion-beam etch (indicated approximately in direction by the arrows) leaving a portion (30A) remaining beneath the plated pole portion (40) and also leaving the WG layer (20) shaped as shown. Referring next to FIG. 1e, there is shown the fabrication of FIG. 1d wherein a portion of the WG layer has been removed by an etching process, typically a wet chemical etch, leaving the WG layer undercut as shown (20A). This undercut is necessary to eliminate portions of the WG layer that would hinder the progress of the etching operation because of its high etch resistance. It is noted that controlling the formation of the undercut regions is difficult in practice and can introduce additional width variations as the process continues. It is further noted that the undercutting of the WG layer is subsequently responsible for the disadvantageous width variations that the present invention eliminates.
Referring next to FIG. 1f, there is shown the fabrication of FIG. 1e wherein the shield layer (10) has been shaped by a low angle IBE (indicated by arrows) to leave a pedestal (10A) beneath the plated pole portion. Finally, with reference to FIG. 1g, there is seen the fabrication of FIG. 1f wherein a final high angle IBE trim (see arrows) has narrowed the width of the plated pole portion (40A) and the seed layer beneath it (30B) to the desired width. The figure also illustrates the disadvantage of the method in that there is a width discontinuity between W1, the common width of the pole piece and seed layer, and W2, the width of the pedestal portion of the shield (10A). This discontinuity is caused by the non-uniform width of the write gap layer (20A) which shields the pedestal (10A) from the effects of the IBE because the wider base of the write gap layer acts as a mask to block the high angle IBE and prevent thinning of the shield pedestal (10A).
Tao et al. (U.S. Pat. No. 5,874,010) teach a method of pole trimming wherein a mask of nitride-forming refractory metal is deposited on the top of the pole piece to be trimmed and then a energetic beam of nitrogen ions is used to etch away the sides of the pole piece not protected by the mask. Feng et al. (U.S. Pat. No. 5,878,481) teach the formation of a pole piece wherein the WG layer is alumina and wherein an anisotropic reactive ion etch is used which has a 2:1 etch selectivity for the WG layer with respect to the upper pole layer and a 1:1 etch selectivity of the lower pole layer with respect to the upper pole layer. Armstrong et al. (U.S. Pat. No. 5,901,432) teach a method of trimming a pole piece wherein the WG layer of nickel phosphorus is initially formed to the correct thickness by electroplating it within a photoresist form. Thus, the ion-beam milling is not required to remove any of the WG layer since it is already of the correct dimensions. Wu et al. (U.S. Pat. No. 6,141,183) teach a method for trimming a pole piece by forming, on a shield and write gap layer, an upper pole piece with laterally extended flanges within a form having already the proper width. The flanges then serve as IBE milling guides to trim the gap and shield layer beneath them.
The prior art cited above does not address the problem associated with the method of pole trimming described in FIGS. 1a-1f. To address that particular problem the present inventors have discovered that the prior art WG material layer composition of alumina must be changed to avoid the differential IBE milling rate that causes the alumina protrusion and leads to the width differential of the pole piece section above and below the WG layer.